However, the only person who has a reason to strip the DRM/embedded data from a file is the file buyer, and I don't think that files are usually illegally distributed by the person who paid for them...

Considering that it's "nearly" technologically impossible for anyone other than "the person who paid for them" to strip today's most prevalent forms of DRM from ebooks ... you may want to rethink that assumption. Unless you think darknet still only contains scanned copies of physical books. Darknet is brimming with copies of ebooks that the original buyer stripped the DRM from and:

A) uploaded/distributed to darknet themselves.

B) passed along to an untrustworthy person who distributed/uploaded it (which they'd still be fiscally/legally responsible for under your proposed system).

But make no mistake: there is no "second-hand DRM-stripping of someone else's purchases and then the mass/illegal distribution of that file" going on in today's scene. At the very best, the original owner is handing a possibly illicit copy of an ebook to a pirate... and at the worst: uploading that illicit copy themselves.